In general, exhaust expelled from an engine of automobiles and the like comprises components such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOX), etc. An exhaust cleaning catalyst to clean out these components from the exhaust is placed in the exhaust path from an internal combustion engine. Such exhaust cleaning catalyst is constructed with a catalyst coating layer formed on a substrate surface. The catalyst coating layer is constituted with a precious metal catalyst and a carrier that carries the precious metal catalyst.
For this type of exhaust cleaning catalyst (especially, gasoline engine exhaust cleaning catalyst), a so-called three-way catalyst is used. In the three-way catalyst, platinum (Pt), rhodium (Rh), palladium (Pd) and the like are used as precious metal catalysts. Among the precious metal catalysts, Pt and Pd contribute to the activity to clean (by oxidation) carbon hydrocarbons (HC) and carbon monoxide (CO) while Rh contributes to the activity to clean (by reduction) nitrogen oxides (NOX).
The three-way catalyst is designed to effectively work in an exhaust gas atmosphere (or a “stoichiometric atmosphere” hereinafter) resulting from near-stoichiometric (theoretical) air-fuel ratio combustion. Thus, in richer exhaust (referring to exhaust generated from combustion of a rich air-fuel mixture) than this as well as in the opposite, lean exhaust (referring to exhaust generated from combustion of a lean air-fuel mixture), the activity of the three-way catalyst is lower than in the stoichiometric atmosphere.
Accordingly, to adjust (buffer) the atmosphere in the exhaust cleaning catalyst, as the carrier, an oxygen-storing/releasing material (or OSC (oxygen storage capacity) material hereinafter) comprising CeO2 is used. The OSC material absorbs oxygen in oxygen-rich exhaust relative to the stoichiometric atmosphere (or a “lean atmosphere” hereinafter) and releases oxygen in exhaust generated from combustion of excess fuel with less oxygen (or a “rich atmosphere” hereinafter). Thus, it is effective in stably maintaining the exhaust in the catalyst as a stoichiometric atmosphere.
Examples of conventional art literatures related to such exhaust cleaning catalyst include Patent Documents 1 and 2. The exhaust cleaning catalyst according to Patent Document 1 is formed on a substrate and has a first catalyst coating layer that comprises precious metal catalysts such as Pt and Pd as well as an OSC material such as CeO2. It also comprises a Rh-containing second catalyst coating layer in an area in the first catalyst coating layer. The first catalyst coating layer has a first-stage portion with a higher OSC content and a second-stage portion with a lower OSC content. According to Patent Document 1, an exhaust cleaning catalyst having such construction, great NOX-reducing (cleaning) activities can be obtained during normal traveling.